I've just read two fascinating papers in evolutionary biology and psychology on the nature of friendship:
Trivers argues that there are situations in which reciprocal altruism (call it tit for tat, helping each other, even reciprocal benevolence if the word "altruism" scares you off) would be expected to evolve, especially among long-lived animals with long periods of parental care and low dispersal rates who exist in small, mutually-dependent, relatively stable social groups (this description well fits certain primates, especially humans). Tooby and Cosmides build on and extend Trivers's analysis by focusing on the overall system of reciprocal helping as well as the many psychological subtleties that one would expect to emerge in the context of such a system when the actors are highly intelligent, emotionally sensitive, and good communicators (humans yet again). Fascinating stuff. I'll try to blog more about these two essays once I've absorbed them more fully.
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